Recently, laser ignition devices have been studied, with regard to various issues, which use a semiconductor laser as an excited laser source for less ignitable internal combustion engines, such as cogeneration gas fuel engines or lean burn engines, and work to deliver an excited laser, as emitted from the semiconductor laser, to a Q-switched laser resonator to create a high-energy density pulsed laser and to focus it on an air-fuel mixture using a light-collecting means to enhance the energy density thereof, thereby igniting the air-fuel mixture.
Such a type of laser ignition device has a laser spark plug with a heat-resisting optical window formed in a top end thereof to protect an optical device such as a condenser lens or a laser resonator from high pressure or intense heat generated in a combustion chamber.
Generally, lubricating oil is put into between a cylinder which defines a combustion chamber of an internal combustion engine and a piston moving up and down within the cylinder in order to reduce friction therebetween. This may cause the lubricating oil to be diffused in the form of a spray within the combustion chamber and transported by a gas flow created in the cylinder together with the air-fuel mixture so that it adheres to the surface of the optical window.
The lubricating oil usually contains non-combustible components such as metal which may be changed into oxides and deposited on the optical window with long-time use, thereby resulting in a reduction in transmitted power of the pulsed laser.
In order alleviate the above problems, Japanese Translation of PCT Internal Application Publication No. 2013-527376 teaches an external laser-induced ignition device which includes an aperture (74) through which a laser beam passes toward a prechamber (110) disposed in an end (381) of a casing (38) closer to a combustion chamber and at least one gas supply port (120) which connects between an internal space (111) of the prechamber and the combustion chamber. The laser-induced ignition device creates a fluid flow (F) entering the aperture at a minimum angle ε of 45° to a longitudinal axis of a laser spark plug when the fluid flow passes through the gas supply port and then flows into the prechamber.